1. Field of the Invention
The present invention relates to a light-emitting element in which an organic compound is provided between a pair of electrodes and the organic compound exhibits luminescence by application of electric field. Further, the present invention relates to a light-emitting device having such a light-emitting element. Furthermore, the present invention relates to an electronic device and a lighting device having such a light-emitting device.
2. Description of the Related Art
Light-emitting elements, which use organic compounds as light emitters and have features of thinness, lightweight, fast response, and direct current low voltage driving, are expected to be applied to next-generation flat panel displays. In particular, display devices having light-emitting elements arranged in matrix are considered to be superior to conventional liquid crystal display devices, because they have wide viewing angle and excellent visibility.
A light emission mechanism of a light-emitting element is described below: when voltage is applied to a pair of electrodes with an EL layer including a light emitter interposed therebetween, electrons injected from a cathode and holes injected from an anode are recombined at an emission center in the EL layer to form molecular exciton, and energy is released when the molecular exciton relaxes to the ground state and thus light is emitted. An excited singlet state and an excited triplet state are known as an excited state, and it is thought that light can be emitted through either state.
As for such a light-emitting element, improvement of an element structure, a development of a material, and the like have been conducted in order to improve element characteristics.
For example, it is reported that an organic compound included in an electron-injecting layer formed in contact with a cathode, is doped with a metal having a low work function, such as an alkali metal, an alkaline earth metal or a rare earth metal, so that injection barrier can be reduced in injection of electrons from the cathode into the electron-injecting layer including an organic compound and thus driving voltage can be reduced (e.g., see Patent Document 1).
Further, it is also reported that an optical adjustment of an emission spectrum can be conducted without increase of a driving voltage, in relation to the above-described technology (e.g., see Patent Document 2).
Specifically, between a cathode and an EL layer in a light-emitting element, a layer in which a hole-transporting organic compound is doped with a metal oxide is formed in contact with the cathode, and a layer in which an electron-transporting organic compound is doped with a metal having a low work function, such as an alkali metal, an alkaline earth metal, or a rare earth metal, is formed so as to be in contact with the layer doped with metal oxide, and the thickness of the layer doped with metal oxide is increased to conduct an optical adjustment of an emission spectrum. In this case, because the hole-transporting organic compound has higher carrier mobility than the electron-transporting organic compound, an increase of driving voltage can be more suppressed than in a case where the thickness of the layer in which the electron-transporting organic compound is doped with a metal having low work function is increased.